The importance of recycling processes in the Middle Miocene Badenian evaporite basin (Carpathian foredeep): palaeoenvironmental implications

Abstract

The Middle Miocene Badenian evaporitic succession within the Carpathian foredeep (Poland) has been conventionally described as formed in a purely marine basin. Until recently, data from this basin have been used to deduce the sulphate isotopic composition of the ocean during the Miocene and to describe evolution of major elements in seawater. This paper presents a new interpretation proving major continental inputs and ongoing recycling processes. It is based on detailed petrographical/mineralogical studies, X-ray microanalysis of primary fluid inclusions in halite (Cryo-Scanning Electron Microscopy (SEM)–Energy Dispersive Microanalysis (EDS)), isotopic analysis (δ34S and δ18O in sulphates) and computer-based evaporation models. Two boreholes, Wojnicz intersecting the halite succession in the centre of the basin and Cieszanów borehole cutting through the gypsum units found in the margin of the basin, are used in reconstructing the general palaeohydrological evolution of the basin. Model calculations constrained from experimental data suggest recharge proportions during halite precipitation of between 20−30% seawater and 65−70% continental waters, with 5−10% of the continental waters recycling previously precipitated halite. The isotope signatures measured show average values of δ34SCDT=+24.61‰ (±0.34) and δ18OSMOW=+12.06‰ (±0.40) for Wojnicz borehole and δ34SCDT=+22.74‰ (±0.42) and δ18OSMOW=+12.66‰ (±0.38) for the Cieszanów borehole. These are consistent with an ocean having a sulphate isotope composition similar to Eocene or present day and the scenario of intrabasin recycling processes of previous marine halite successions. The present interpretation for the Carpathian foredeep evaporites argues against the use of these in deducing Miocene seawater composition.